The present invention relates generally to magnetic toner retaining means for removing excess toner from a photoconductive member such as an electrostatically imaged lithographic master sheet and, particularly, to magnetic means for retaining excess toner at a development zone to prevent its being carried away by the master sheet and contaminating the reproduction equipment.
In the lithographic printing and duplicating field, various types of planographic printing plates or masters are used for reproducing copies. A master may receive a direct image from a typewriter or the like, a photographic image on a suitably sensitized surface or an electrostatic image on a photoconductive surface, normally comprising zinc oxide. In the latter system, the photoconductive surface of the master is exposed to an original to be copied via an optical projection system to produce an electrostatic latent image which is subsequently developed with a magnetic toner to produce a visible image. The developed master may be treated with a conversion solution to provide a lithographic surface for producing offset lithographic copies in a conventional manner.
Difficulty is often experienced in keeping the master sheet free of excess toner deposited during the development process which toner works its way to the underside of the sheet. The excess toner may be carried out of the development zone by the master sheet resulting in lost toner, and of even more concern, contamination of the reproduction system. Usually, the unwanted toner deposits accumulate along the margins of the master sheet, especially where the magnetic roller brush for applying toner to the master is longer than the width of the master.
There are many known devices such as filters, toner traps and cleaning rollers which are used to help reduce toner contamination of the reproduction equipment and surrounding areas. Such devices are generally complex, expensive or of limited capacity.